Ultra-lightweight, Deployable 1m-Class Optical Telescope for SSA Applications

RC Romeo1, RN Martin1 SR Restaino2, CC Wilcox2, JR Andrews2,

T Martinez2, F Santiago2, J Clark2, J Walton2, SW Teare2, 3 , DM Payne4,

E Penado5, P Wood5

1 Composite Mirror Applications 2 Naval Research Laboratory

3 Department of Electrical Engineering, New Mexico Tech 4 Narrascape, Inc.

5 Northern Arizona University

1. ABSTRACT

Deployable Optical/IR telescopes must be lightweight and rugged enough to withstand being placed from one location to another, in mostly adverse environments. This requires unique materials that are light, stiff and dimensionally stable in most any environment including a space environment. Continuous fiber reinforced polymer (CFRP) composite materials have been used to produce ultra-lightweight optical telescopes. Their thermal/mechanical properties offer advantages over conventional materials in terms of thermal expansion, stiffness and strength. The anisotropic nature of the material allows for weight and stiffness-optimized designs of dimensionally stable structures, ideal for telescopes. Optical mirrors can be produced from CFRP composites as well; yielding 1m diameter rigid primary mirrors weighing 12 kg. Presented will be unique telescope and mirror fabrication leading to extremely lightweight OTA systems up to 1.4m for the Navy Prototype Optical Interferometer, NPOI. The 1.4m NPOI Telescope, being produced by CMA, is required to be deployable along the baselines of the interferometer. We present the design of this system and how it evolved from various deployable concepts for optical telescopes.

2. CFRP MATERIAL ADVANTANGES

Carbon Fiber Reinforced Polymer (CFRP) composite materials offer many of the same advantages of SiC and Beryllium and certainly greater advantages over more conventional telescope materials. Unidirectional laminates are appropriate for low CTE structures such as strut tubes. Multi-angle laminates are used to produce plate elements with quasi-isotropic behavior. CFRP material allows for tailoring properties of structures and mirrors. See figure X for graphic representation of ply-orientations. The following are advantages for using CFRP materials to produce lightweight, deployable optical systems.

Low Mass Density of CFRP materials is 1.6 gm/cc, which is roughly half of that of aluminum. Low mass produces low inertial problems that can result in high deflections of the optics during transportation and deployment. Obviously, low mass results in low energy requirements and fewer inertial problems affecting fast tracking and slewing.

Low Thermal Expansion CFRP materials exhibit low coefficient of thermal expansion (CTE), lower than invar. CFRP telescopes and optics are produced from the same materials eliminating CTE mismatch resulting in simple telescope designs. Eliminating CTE mismatch can eliminate the need for costly and complex flexure and mirror cell designs. Low CTE designs such as unidirectional oriented laminates (unidirectional strut tubes or plates) along the optical axis will minimize the need for focus change across the operating temperature range of the system.

Anisotropic Design Flexibility The unidirectional properties of CFRP material enables biasing of mechanical and thermal properties radially in plane. This means that ideal properties can be applied in specific directions in-plane, offering design flexibility while minimizing mass in directions on the structure where it is not needed. See figure 1a,b. Sandwich panels as shown in figure 2 offer lightweight and stiff structures, a great example of design flexibility.

High Stiffness/Strength-to-Weight Ratio CFRP materials typically have stiffness-to-weight ratio 4 times greater than Aluminum, Cr Steel and Titanium. This makes CFRP materials ideal for stiff lightweight telescope structures, especially deployable systems that rely on the stiffness to hold alignment of the optics during deployability and under operating conditions at various elevation angles. See figure 3.

Figure 2 Honeycomb Sandwich Construction

Figure 1a Multi-Angle Plies

Figure 1b Unidirectional Plies

Figure 3 Stiffness/strength-to-weight ratios of CFRP composites compared to other

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3. CFRP MIRRORS 3.1 Composite Mirror History

CFRP composites have a history mixed history of success across a wide range of applications. JPLs Precision Segmented Reflector (PSR) program involved using carbon fiber/polymer matrix composite materials to produce image quality optics for use at submillimeter and into IR wavelengths down to 10.6m, with a long term goal into visible wavelengths. The program was concluded with mixed results. In fact, no visible wavelength mirrors were produced under PSR due to complexity of processing and immaturity of high performance, dimensionally stable composite materials. The PSR program, however, did yield vast amounts of material property data relating to composite materials, carbon fibers and various polymers such as epoxies and cyanate esters. Thermal performance data was also obtained for these materials suggesting their possible use for space telescope applications. CTE studies of the composites were measured to determine preferred ply-orientations, which could yield low stress and low out of plane deflections in the composite mirror substrates during processing. In addition, Sandia National Labs had performed studies relating internal stresses and resulting out-of-plane deflections of laminated composites due to ply-orientation errors [1]. This effort was important for verifying the use of cfrp composites for lightweight mirror substrates, but more work was required to produce specular mirrors. The key issues remaining were producing a highly polished surface quality and releasing the mirror from the mandrel. CMA has progressed to yield near diffraction-limited optical performance at 10X reductions in Mass. 3.2 CFRP Mirror Fabrication

CFRP mirrors are produced via replication, which follows the path in figure 4 below. One must begin the replication process with a mold or mandrel exhibiting the identical optical prescription and accuracy of the desired final CFRP mirror. Bear in mind, the resulting CFRP mirror will only be as accurate as the mandrel unless some active figure control is applied to the mirror. The CFRP material is laid-up in a specific ply-orientation on the mandrel and the entire system is processed under heat and pressure. The result is a finished mirror requiring no grinding or polishing, ready for optical coating, again, as shown in figure 4. Figure 4 CFRP fabrication sequence from left to right, processing the glass mandrel, CFRP prepreg material, lay-

up of prepreg over the glass mandrel, processing and release CFRP mirror from the mandrel, ready for coating.

3.3 CFRP Mirror Optical Performance Currently, composite mirrors can be produced to near diffraction-limited optical performance at visible wavelengths. CMA is continuing development of the optical performance of CFRP mirrors under contract with the NRL supporting the Navy Optical Prototype interferometer (NPOI) program. In particular, CMA is producing a 1.4m prototype deployable telescope that includes CFRP primary and secondary mirrors. Smaller optics, 0.4m aspheric, were used as test coupons to establish procedures and materials for producing the 1.4m passive, f/2.0 parabolic primary mirror for the NPOI system.

The images below show interferograms and data reduction indicating imaging quality optics produced for test articles. The surfaces were achieved without grinding or polishing. Figure 5 shows ZYGO results of early work on a 150mm spherical mirror, indicating near diffraction-limited performance. The mandrel accuracy used for this mirror was /4 p-v ( = 633 nm) on the wavefront. Figure 6 shows optical data relating to the 0.4m test mirrors. The mirrors in this case are limited by the wavefront quality of the mandrel [2], which measures to 0.5 p-v. A 0.4m

Cassegrain primary is shown in figure 7. This mirror was produced for the NRL and the 0.4m prototype telescope described in section 4.1.

Figure 5 ZYGO interferograms on a 150mm f/6.2 spherical mirror, mirror photo shown inset. Wavefront measures 0.28 p-v. Mirror was produced in 2001 and indicates decent optical performance.

Figure 6 (Left) 0.4m parabolic CFRP mirror with aluminum coating, (Center) interference fringes for the mirror, (Right) 50lines/inch Ronchigram for mirror. Note turn-up edge [3] and central divot, indicating mandrel features.

4. OPTICAL TUBE ASSEMBLY DESIGN The compatibility of the CFRP composite materials for use in deployable structures and CFRP mirrors has been defined above. The design of optical tube assemblies (OTA) from CFRP materials requires an understanding of how to produce the desired properties in the proper locations on the OTA. This means an understanding of when to adopt and when not to adopt unidirectional ply-orientation laminates. Conversely, it may be simpler to produce CFRP parts from quasi-isotropic ply orientations to avoid non-uniform thermal/mechanical properties but a weight penalty is paid as well as producing greater-than-zero CTE structures. An intelligent CFRP OTA design will incorporate both types of laminates as shown in figures 1a and 1b. Deployable systems are not only concerned with dimensional stability and low mass, they must also be designed to withstand the load cases applied during stowage, transportation, deployment and redeployment. This requires the proper material selection capable of the strength and toughness required to meet all loading conditions. Fabrication methods will also be a key facto